4 Discussion
A growing body of studies have highlighted phenological shifts in plants in response to global change factors (Cleland et al., 2006; Zhou et al., 2023; Yang et al., 2023). Nevertheless, how species-specific responses impact plant community dynamics remains poorly understood (CaraDonna & Inouye, 2015). Our study explored how long-term warming and nitrogen addition affected plant flowering phenology and community dynamics in a temperate desert steppe. Our results documented the previously unrecognized responses of plant flowering phenology and species dominance to warming over ten consecutive years. We find that warming advanced plant flowering time by 4.3 days for C3 plants and 2.8 days for C4 plants, respectively. Warming resulted in reductions in the duration of flowering and consequent declines in the dominance of C3 plants. In contrast, warming did not change the duration of flowering of C4plants, but enhanced their dominance. The increased dominance of C4 plants was further favored by N deposition which extended the duration of flowering of C4 but not C3 plants. To our knowledge, our study is the first to report temporal variation in C3 and C4plants flowering phenology in a desert steppe. Importantly, these warming-induced changes in flowering phenology led to changes in community composition via plant dominance, providing empirical evidence that environmental change can alter community dynamics via modifying plant phenology.
An important question is what caused the variation in the warming effect on flowering phenology of C3 and C4plants across ten years of the experiment. Our results revealed that warming significantly advanced the flowering time of five species and shortened the duration of flowering of most plants. This may be attributed to accumulated temperature for plant growth and development (Zhang et al., 2007; Forrest, 2015), whereby an increase in temperature could induce a shift from vegetative to reproductive growth of plants (Zhang et al., 2003; Liu et al., 2021). Moreover, the results of SEM analysis indicated that soil temperature warming affects flowering phenology. Specifically, warming increased the 0-30cm soil temperature and shortened the duration of flowering of C3 plants. This agrees with a previous study that reported that soil surface warming shortened the flowering phenology of the dominant C3 grass, S. breviflora (Bai et al., 2022).
Several mechanisms can potentially explain the observed changes in flowering phenology with warming. First, warming can alter plant physiology through the mediation of plant enzyme and hormone activity, which in turn influences plant phenology (Weih & Karlsson, 2001; Crimmins et al., 2010; Fanin et al., 2022). For instance, increased temperature has been shown to prolong the period of vegetative phenology by reducing production of ethylene and enhancing leaf lifespan (Moore et al., 2021; Ren et al., 2021), but shortened the period of reproductive phenology (Fracheboud et al., 2009; Zettlemoyer et al., 2019). Second, warming can alter flowering phenology by affecting leaf water loss (Weih et al., 2001). Increased temperature can accelerate vegetative growth by affecting the rate of leaf water loss and reduced the period of flowering phenology (Pallas et al., 1967; Sadok et al., 2021). Third, warming can influence pollinators that consequently result in phenological changes (Shivanna & Tandon, 2014). Insect-pollinated species that flower early have been found to be more sensitive to warming than those that flower later (Fitter & Fitter, 2002). Finally, warming can affect flowering phenology by regulating plant traits associated with resource acquisition. For example, warming-induced soil surface water loss has been shown to limit the growth and development of shallow rooted plants and shorten their duration of flowering (Wang et al., 2021).
Our results show that N addition prolonged the duration of flowering of C4 plants, but did not affect that of C3plants. SEM showed that the N effect on flowering phenology was mediated by soil water availability. High N supply has been shown to affect soil porosity and soil structure by increasing litter content, promote the retention and migration of soil water and nutrients, and thereby enhance soil water availability (Prevéy & Seastedt, 2014; Richardson et al., 2018). In parallel, N addition also resulted in increased N uptake by plant roots. This suggests that, from a resource allocation perspective, more resources will be allocated to reproductive growth, leading to an extension of the time needed for the development of reproductive organs and consequent increases in duration of flowering (Tilman & Wedin, 1991; Xia & Wan, 2013).
We further find that duration of flowering had a consistently positive effect on species dominance under warming and N deposition. Several potential factors were likely to explain the relationship between plant phenology and community dynamics, such as height, density and coverage of plants. First, warming significantly reduced height of C3 plants (Figure S6), which probably led to decreases in light availability (Sun & Frelich, 2011; Liu et al., 2022), and thus shortened duration of flowering and decreased dominance of C3 plants. In contrast to the response of C3 plants to warming, a longer duration of flowering of C4 plants could allow more time for photosynthetic activity and nutrient acquisition (CaraDonna & Inouye, 2015; Chen et al., 2020), and consequently enhance the dominance of C4plants in a community. Second, N addition increased duration of flowering, coverage and density of C4 plants(Figure S6), which could contribute to the increase of dominance of C4 plants in a community (Myneni et al.,1997; Jiang et al., 2016). An extended duration of flowering can enhance species’ competitive ability, seed yield and reproductive success (Craufurd & Wheeler, 2009; Ernakovich et al., 2014; Godoy & Levine, 2014), and thus increase species dominance in a community.
Current attempts to link plant phenological responses to global change have primarily focused on shifts in ecosystem functioning (Wolf et al., 2017; Wang et al., 2020; Liu et al., 2022), but often neglect possible effects on plant community dynamics. Our results emphasize the importance of flowering phenology in mediating community dynamics under climate change. We found that warming-induced changes in the duration of flowering have a large effect on plant dominance, with duration of flowering extensions of C4 plants increasing their dominance, while decreases in duration of flowerings contribute to declines in the dominance of C3 plants. These findings suggest that plant flowering phenology plays an important role in modulating plant dominance and community dynamics in response to global change. The present study provides empirical evidence that variation in phenological responses among plants influences species dominance, ultimately driving shifts from C3 to C4plants under increased temperature and N deposition. These changes in plant phenology and dominance are likely regulated by soil temperature and water availability. Our results highlight the importance of including plant phenology to better understand the mechanisms that drive changes in community structure and ecosystem function under climate change.